In the oil and gas exploration and production industry, a wellbore or borehole of an oil or gas well is typically drilled from surface to a first depth and lined with a steel casing which is cemented in place. The borehole is then extended and a further section of tubing known as a liner is located in the borehole, extending from the casing to a producing formation, and is also cemented in place. The well is then completed by locating a string of production tubing within the casing/liner, through which well fluids flow to surface.
However, before the well can be completed, it is necessary to clean the lined wellbore and replace the fluids present in the wellbore with a completion fluid such as brine. The cleaning process serves to remove solids adhered to the wall of the casing or liner, typically by use of scraping and brushing tools; to circulate residual drilling mud and other fluids out of the wellbore; and to filter out solids present in the wellbore fluid. A considerable amount of loose debris accumulates in the wellbore and on the surface of the casing/liner which debris comprises rust particles and metal shavings, cuttings or scrapings originating from equipment used in the well and the casing or liner itself. Whereas some of the debris can be removed during the normal circulation of fluid through the work string or drill string, a significant amount remains, and this is problematic because it may interfere with use of tools and instruments in the wellbore, and consequently efforts have to be made to remove such residual debris.
In an effort to address the metallic debris issues, magnetic well cleaning apparatus has been developed, such as that disclosed in the Applicant's UK Patent Number 2,350,632, which tool includes a number of magnets located under a protective sleeve. Another magnetic fishing tool is described in U.S. Pat. No. 6,591,117, wherein, large bar magnets are spaced apart around and along a tool body for the purposes of attracting and retrieving metal debris. These magnets may be permanent magnets made of any suitable magnetic material, including rare earth magnets such as neodymium iron boron, ceramic ferrite, samarium cobalt, or aluminium nickel cobalt. The bar magnets are fitted into recesses in the tool body and arranged to have an area between each magnet for metallic debris to settle. A further such tool is described in U.S. Pat. No. 6,354,386, wherein arcuate magnet assemblies are detachably secured by screws or other similar means to a body to be mounted in a drill string. An alternative fastening arrangement described there for the magnet assemblies uses split retainer rings provided with locking members for securing the magnets on the body.
In use of such fishing tools, ferrous metallic particles and debris present in the wellbore is attracted to the magnets and carried out of the wellbore when the cleaning tool is removed or “tripped” from the well.
An object of the invention is to provide further improvements in tool assembly and design.